Balancing between environment conservation and economic development, the renewable energy conversion device that harnesses natural phenomena, such as wind power, hydro power and solar power, etc., for generating electricity is becoming a focal point of any economy whose every fiber vibrates with the logic of cheap oil and careless pollution. For utilizing the wind energy, most conventional large-sized wind turbines in the world use a so-called three phase asynchronous generator, also called an induction generator (IG) to generate AC electricity. While connecting one such IG directly to an electrical grid, its generator slip is usually within −2% and thus it can be considered as operating at fixed rotation speed. One reason for choosing this type of generator is that it is very reliable, and tends to be comparatively inexpensive. However, for most of the time it can not obtain the optimal efficiency on wind energy conversion and has a transmission system that is easily to be affected by gusty wind, moreover, it requires a set of external capacitors to provide the reactive power for supporting the grid voltage.
As variable speed generators can be freed from the aforesaid shortcomings of fixed speed induction generators, they are generally adopted and used in large-sized MW scale wind energy conversion device. The generators suitable for variable speed operation include squirrel-cage induction generators, would-rotor induction generators and synchronous generators. However power converters are required for enabling variable speed control and thus they are comparatively more expensive.
A variable-speed induction generator usually pairs with a back-to-back pulse-width modulation (PWM) power converter so as to enable the same to match with the voltage and frequency of an electrical gird connected thereto. Furthermore, if a squirrel-cage induction generator (SCIG) is used, a full power converter is required; and if a wound-rotor induction generator, being also called doubly-fed induction generator (DFIG), is used, it only requires a partial power converter of usually one third power rating of a full power converter so that it is comparatively much cost-effective and thus it is the mainstream design and occupies about 70% market share of large-sized wind power generating device (wind turbine). In addition, in consideration of magnetizing current, a multi-pole structure is not suitable for the induction generators and thus it is necessary to pair the induction generator with a gearbox so that the shaft speed can be compatible with the synchronous speed of the induction generator.
For a synchronous generator used in a variable speed wind turbine, there is a future trend to adopt multi-pole permanent magnet synchronous generators (PMSG) for reducing copper loss. As the PMSG can be easily adapted for a gearless design that the drive train can be simplified and thus the energy conversion efficiency is enhanced. However for the large sized wind turbines, since most PMSGs are custom-made and each requires to be paired with a full power converter, it is still very expensive.
From the above description relating to the performance of DFIG and PMSG, there pros and cons can be summed up as following:
Regarding to the doubly-fed induction generator (DFIG):
                (1) Since it requires a gearbox for speed increasing and its rotor needs to be configured with slip rings, regular maintenance is necessitated.        (2) As its practical slip is in the range of ±30%, the cut-in speed of a wind-turbine applying the same is limited by its positive maximum slip.        (3) As it adopts a partial power converter and its stators is connected to an electrical grid that causes the decoupling from the electrical grid to be impossible when the wind power generating device (wind turbine) is operating, the wind power generating device (wind turbine) can not maintain it normal operation when the electrical gird is subjected to a sudden voltage dip. Hence, it is difficult to abide by the transmission system operator's request to maintain the wind power generating device (wind turbine) on the grid and output the reactive power for assisting the voltage recovery during a faulted condition.        (4) As the magnetizing current required thereby is supplied by the grid, it is not a stand along device that can operate independently at a remote area, such as an isolated island.Regarding to the synchronous generator (SG):        (1) Since it can fit to a gearless design, it is possible that the gearbox maintenance is not required, so that it is more reliable. However, as the gearless design will cause a larger torque on the generator that its diameter is comparatively larger, a high power synchronous generator is bulky and thus not easy for transportation and installation.        (2) As it is supported by a full power converter and its rotation speed can be controlled more widely, the wind power generating device (wind turbine) using the same can feature a lower cut-in speed.        (3) As it is supported by a full power converter so that the wind power generating device (wind turbine) using the same can be decoupled from the electrical grid connected thereto, the wind power generating device (wind turbine) is capable of maintaining its normal operation even when the electrical grid is suffering a sudden voltage dip. Hence, it is feasible to abide by the transmission system operator's request to maintain the wind power generating device (wind turbine) on the grid and output the reactive power for assisting the voltage recovery during a faulted condition.        (4) Since it can generate electricity independent of the condition of the grid when the rotor is being driven by the wind, it can be a stand along device that is suitable to operate independently at a remote area, such as an isolated island.        
From the above description, it is noted that the DFIG is inferior comparing to the SG in the respect of fully decoupling with the grid. However, comparing to DFIG, the SG is more expensive and bulky.
Currently, most wind power generating devices (wind turbines) only utilize a single generator, thus the performance of the wind power generating device (wind turbine) is restricted by the characteristics of that single generator as stated above and can not be improved by integrating such generator with advantageous characteristics of other generators. For those wind power generating devices (wind turbines) with multiple generators, the design idea is either to adopt a big-small pair for fitting to different wind speed conditions, or to use a plurality of generators of same but smaller rated power to obtain a bigger resultant power so that the benefits of more cost effectiveness and redundancy can be secured.
To name a few such wind power generating device (wind turbine)s with multiple generators, the V47-660 kW of Vestas, Denmark, and the Liberty-2.5 MW of Clipper, U.S.A. can be the representatives. The V47-660 kW of Vestas is structured as the one shown in FIG. 1, which is a conventional fixed speed wind power generating device (wind turbine), being primarily comprised of: a primary electrical generator 9 and an auxiliary electrical generator 16. As the rated power of the auxiliary electrical generator is defined to be smaller than that of the primary electrical generator, the operation principle of the wind power generating device (wind turbine) of FIG. 1 is specified as that: the auxiliary electrical generator 16 is enabled when it is subjected to a condition of low wind speed or is at its initial operating stage; and the primary electrical generator 9 is enabled when the wind speed exceeds a specific speed. Furthermore, the Liberty-2.5MW of Clipper is structured as the one shown in FIG. 2. The wind power generating device (wind turbine) of FIG. 2 adopts a design disclosed in U.S. Pat. No. 6,304,002, entitled “Distributed Powertrain for High Torque, Low Electric Power Generator”, which is comprised of a plurality of small-sized electrical generators 70 of the same rated power, whereas each electrical generator 70 can be a SCIG or a PMSG. Such wind power generating device (wind turbine) not only can be handled and assembled easily, but also it has excellent redundancy. Nevertheless, those plural electrical generators used in the wind power generating device (wind turbine) are still of the same operation principle, so that the performance of the wind power generating device (wind turbine) is still restricted by the characteristics of that particular type of generator.
Please refer to FIG. 3, which shows a wind power generating facility of Matsuo Bridge Co. Ltd., disclosed in Japan Kokai Tokkyo Koho No. 9-60575. The wind power generating facility of FIG. 3 is comprised of a primary wind turbine (unit 1) and an auxiliary wind turbine (unit 2), wherein the primary wind turbine includes a self-exciting induction generator 3 and the auxiliary wind turbine includes a synchronous generator 4. When an electrical grid connecting to the aforesaid wind power generating facility is working normally for supplying electricity, the primary wind turbine (unit 1) can acquire a magnetizing current from an electrical grid and thus feed electricity generated therefrom to the electrical grid. On the other hand, the auxiliary wind power turbine (unit 2) is enabled to provide reactive power when the electrical grid is out or the wind power generating facility is working independently. Nevertheless, since the auxiliary wind turbine (unit 2) is acting just as a backup wind turbine, so that the two wind turbines (units 1 and 2) are not being enabled simultaneously for boosting the performance of the wind power generating facility.
Please refer to FIG. 4 and FIG. 5, which are respectively a longitudinal sectional view of an AC motor used in a variable speed driving apparatus disclosed in U.S. Pat. No. 5,365,153, entitled “AC Variable Speed Driving Apparatus and Electric Vehicle using the same”, and a block diagram showing a driving circuit of the AC motor of FIG. 4. In the AC motor M of FIG. 4, the reference numeral 10 designates a frame in which a permanent magnet synchronous motor 20 and an induction motor 30 are incorporated having a common axis of rotation 11. The permanent magnet synchronous motor 20 is connected to an inverter 28 through three phase terminals 24 while the induction motor 30 is connected to an inverter 38 through its three phase terminals 34. In the driving circuit of the AC motor M shown in FIG. 5, the main battery 1 is connected to voltage type inverters 28 and 38 through DC disconnecting switches 27 and 37, respectively, while the control system 45 is capable of detecting the status of the semiconductor device groups 25 and 35 and controlling the on/off of the two switches 27 and 37. Hence, an electric vehicle using the AC motor M can get the benefits of both the synchronous motor 20 and the induction motor 30, each operating at different rotation speed with respect to its torque output characteristic, and thus the AC motor M is considered to be highly efficient in a wide speed range. However, this patent is related to motoring mode operation of electrical machines, not generation mode.